Process for liquefying hydrogen

ABSTRACT

The invention relates to a process for liquefying hydrogen. To reduce the specific energy consumption, the following process steps are used: a) the precooling of the hydrogen stream by indirect heat exchange against a pressurized LNG stream to a temperature of between 140 and 130 K, b) the precooling of the hydrogen stream by indirect heat exchange against a coolant to a temperature of between 85 and 75 K, c) where the precooling of the coolant takes place against a pressurized LNG stream, and d) the cooling and at least partial liquefaction of the precooled hydrogen stream takes place by indirect heat exchange against another hydrogen stream channeled through a closed cooling circuit, e) where the precooling of the condensed hydrogen stream, which is channeled through a closed cooling circuit, takes place against a pressurized LNG stream.

The invention relates to a method for liquefying hydrogen.

Hydrogen in particular is currently increasingly gaining in importance as energy carrier due to the growing energy demand and increased environmental consciousness. Trucks, buses, passenger cars and locomotives are thus already powered by means of engines which are operated by natural gas or hydrogen as well as by means of combinations of fuel cell and electric motor. In those cases, the most sensible form of storage of the hydrogen “on board” the aforementioned means of transportation is the liquid form. Even though, for this purpose, the hydrogen must be cooled to approximately 25 K and maintained at this temperature—which is only achievable by using appropriate insulation measures on the storage containers or storage tanks—, owing to the low density of GH₂, storage in gaseous form in the aforementioned means of transportation is, as a rule, less favorable, since, in this case, storage has to take place in large-volume and heavy storage tanks under high pressures.

Hydrogen liquefaction processes normally comprise two process steps, namely the so-called precooling step as well as the subsequent liquefaction step. In the above processes, hydrogen must be cooled to below its upper Joule-Thomson inversion temperature—this is understood to be the temperature below which an expanding gas cools down—before it can be liquefied. The hydrogen must therefore usually be precooled to a temperature of at least −150° C. before it can be supplied to the subsequent liquefaction process.

Gaseous hydrogen is usually composed of approximately 75% ortho-hydrogen and approximately 25% para-hydrogen. For this reason, during the liquefaction process—since the liquefied hydrogen is normally to be intermediately stored over a longer period of time—, the ortho-hydrogen must be converted into para-hydrogen. Typically, a proportion of at least 99% para-hydrogen is aimed for. If such a conversion is not performed, a quicker evaporation of the liquefied hydrogen will be the result. The conversion from ortho-hydrogen to para-hydrogen takes place by means of suitable conversion catalysts.

A large number of methods for liquefying hydrogen are known from the literature, in which the precooling of the gaseous hydrogen takes place against a coolant circuit or coolant mixture circuit. Nitrogen is often used as coolant in this case. Hydrogen liquefaction methods are known from the international patent application WO 2005/080892 as well as from the European patent application 1 580 506, where the precooling of the hydrogen stream to be liquefied takes place in indirect heat exchange with a pressurized LNG (Liquid Natural Gas) stream. The LNG evaporating during this process transfers its cold to the gaseous hydrogen stream to be precooled. The evaporation of LNG is an issue in particular in LNG terminals. This evaporation normally takes place by means of suitable natural gas burners which are immersed in water baths and are operated with a small partial stream of the LNG.

It is the object of the present invention to provide a method for liquefying hydrogen, which, compared to the methods which form part of the state of the art, has a lower specific energy consumption.

The method according to the invention for liquefying hydrogen comprises the following method steps:

-   -   a) precooling of the hydrogen stream by indirect heat exchange         against a pressurized LNG stream to a temperature of between 140         and 130 K,     -   b) precooling of the hydrogen stream by indirect heat exchange         against a coolant to a temperature of between 85 and 75 K,     -   c) with the precooling of the coolant taking place against a         pressurized LNG stream, and     -   d) cooling and at least partial liquefaction of the precooled         hydrogen stream by indirect heat exchange against a further         hydrogen stream which is circulated in a closed cooling circuit,     -   e) with the precooling of the compressed hydrogen stream, which         is circulated in a closed cooling circuit, taking place against         a pressurized LNG stream.

The method according to the invention for liquefying hydrogen will be explained in more detail below with reference to the exemplary embodiment illustrated in the FIGURE.

The hydrogen stream to be liquefied is supplied via line 1 with a pressure of 2200 kPa and a temperature of 300 K to the heat exchanger E1. In the latter, the hydrogen stream is cooled to a temperature of 135 K against an LNG stream, which is conducted via line A through the heat exchanger E1 and has a temperature of 125 K and a pressure of 7800 kPa.

this case, gaseous hydrogen produced during the intermediate storage of the liquid hydrogen product stream can be supplied to the ejector.

The open hydrogen cooling circuit is composed of the line sections 17, 11, 13, 15 and 16, the heat exchangers E4, E5, E6 and E7, at least one expansion device 12, and a preferably multi-stage compressor 14. Hydrogen is first supplied via line 17 to the heat exchanger E4 and cooled therein. It is subsequently supplied via line 11 to the expansion device 12 and expanded in it for the purpose of providing the peak cold necessary for the liquefaction of the hydrogen.

Next, the evaporation takes place in the heat exchanger E7 and a heating of the expanded hydrogen stream in the heat exchanger E4 in indirect heat exchange with the hydrogen stream to be cooled and liquefied in line 17. The heated hydrogen stream is supplied via line 13 to the heat exchanger E5 and heated against itself therein, prior to being compressed to the desired circuit pressure in the compressor unit 14.

The compressed hydrogen stream is supplied via line 15 to a heat exchanger E6 and cooled therein against a further partial LNG stream, which is supplied to the heat exchanger E6 via line C. This cooled hydrogen stream is subsequently supplied via line 16 to the heat exchanger E5, cooled against itself therein and thereafter supplied again via the line sections 17 to the already described heat exchanger E4.

For reasons of clarity, several expansion devices are not shown in the FIGURE; they are being supplied in each case with cooled partial hydrogen streams from the line sections 17 and 11 and, subsequent to the completed cooling expansion, supplied again to the cooling circuit 13 shown, located upstream of the expansion device 12 (before and/or after E4).

The aforementioned nitrogen cooling circuit used for precooling the natural gas stream to be liquefied by means of the heat exchanger E2, has in addition to the line regions 20, 21, 23 and 24 a further heat exchanger E3, an expansion device 25, as well as a preferably multi-stage compressor unit 22.

The nitrogen stream expanded in the expansion device 25 and having a cooling effect in the process is supplied via line 20 to the aforementioned heat exchanger E2 and heated therein against the hydrogen stream to be cooled, and evaporated. The evaporated nitrogen stream is then supplied via line 21 to the compressor unit 22 and compressed therein to the desired circuit pressure. The compressed nitrogen stream is supplied via line 23 to the heat exchanger E3 and cooled therein against a further LNG stream, which is supplied to the heat exchanger E3 via line B. The cooled nitrogen stream is then supplied via line 24 to the aforementioned expansion device 25.

According to the invention, the LNG being available in the hydrogen liquefaction process environment is now used for precooling the hydrogen stream to be liquefied (heat exchanger E1), for cooling the compressed nitrogen in the nitrogen cooling circuit (heat exchanger E3), as well as for cooling the compressed hydrogen stream (heat exchanger E6) circulating in the open hydrogen cooling circuit.

For reasons of clarity, the catalysts and/or catalyst mountings required for the desired or possibly required ortho-para conversion of the hydrogen are not shown in the FIGURE. Generally, a first ortho-para conversion will be provided downstream of the purification device 4. In this purification device 4, an increase of the para-hydrogen content from approximately 25 to approximately 43% can take place. The following ortho-para conversion takes place preferably by way of catalysts arranged in the passages of the heat exchanger E4. Preferably, the liquid hydrogen product stream withdrawn via line 9 should consist of at least 99% para-hydrogen. 

1. A method for liquefying hydrogen, comprising the following method steps: a) precooling of the hydrogen stream by indirect heat exchange against a pressurized LNG stream to a temperature of between 140 and 130 K, b) precooling of the hydrogen stream by indirect heat exchange against a coolant to a temperature of between 85 and 75 K, c) with the precooling of the coolant taking place against a pressurized LNG stream, and d) cooling and at least partial liquefaction of the precooled hydrogen stream by indirect heat exchange against a further hydrogen stream which is circulated in a closed cooling circuit, e) with the precooling of the compressed hydrogen stream, which is circulated in a closed cooling circuit, taking place against a pressurized LNG stream.
 2. The method as claimed in claim 1, characterized in that nitrogen is used as coolant for the precooling of the hydrogen stream. 